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HTCP Opcodes

HTCP/0.0 defines five opcodes: NOP, TST, MON, SET, and CLR.
Note that ICP has different opcodes for queries and
replies—for example, ICP_OP_QUERY and ICP_OP_MISS. HTCP,
however, uses the same opcode for both messages. The previously
described RR flag indicates whether a given message is a request
or a response. The following sections describe each opcode,
including how each one uses the OP-DATA field.

NOP

The NOP (null operation) opcode is
similar to an ICMP echo or a
“ping” message. It can be used to measure
network delays between caches, as well as simply to find
out if they are alive.

There is no OP-DATA for either
NOP queries or responses, and
the RESPONSE code is always zero.

TST

The TST (test) opcode is similar to the ICP query
operation. It tests for the existence of a cached
response in a neighbor cache.

TST request

For a TST request, the OP-DATA is a SPECIFIER. Recall
that a SPECIFIER includes the HTTP request method, URI,
HTTP version, and the client’s request headers. The
request headers are important because the client may
have certain freshness requirements (Cache-control:
max-age or
max-stale).

TST response

Table D-2 shows the RESPONSE
codes defined for a TST response when F1 is not set.

Table D-2. HTCP TST Response Codes

Code

Description

0

Entity is present in responder’s cache.

1

Entity is not present in responder’s cache.

Note that the RESPONSE code indicates only the presence or absence of the object and says nothing about its freshness. If the cached object ...

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